1. Field of the Invention
The present invention relates to a semiconductor memory device comprising a cell array of SRAM (Static Random Access Memory) cells arranged in matrix.
2. Description of the Related Art
In general, a SRAM cell comprises two PMOS transistors, two NMOS transistors and two transfer transistors: a total of six MOS transistors. The SRAM cell is laid out in an almost point symmetrical pattern in an N-well and P-wells at both sides thereof. More specifically, each transistor has a source and a drain extending in parallel with a boundary between well regions and a gate extending in the direction orthogonal to the above boundary (see, for example, JP 2001-28401A).
In recent years, reductions in device size with developed scaling cause random variations among transistors, which make it difficult to eliminate the tradeoff between the retained cell stability and the improved read/write characteristics with retained fine patterns.
The SRAM cell stability is estimated by the fact that data held in the SRAM cell is not inverted when a transfer transistor turns on while a pair of bit lines are both at “1”. Desirably, the transfer transistor has trans-conductance β(xf) (∝Wxf/Lxf: where Wxf denotes a gate width; Lxf a gate length) smaller than β(pd) of the NMOS transistor. Therefore, the transfer transistor and the NMOS transistor are usually formed in the same impurity-diffused region such that the former has a narrower gate width than the gate width of the latter (for example, JP 2001-28401A, FIG. 1).
On the other hand, an improvement in the read characteristic desires a larger cell current Icell flowing in the transfer transistor and the NMOS transistor. The cell current Icell is almost determined from the trans-conductance β(xf) of the transfer transistor having a lower drive ability than the NMOS transistor. Accordingly, as large β(xf) as possible is desirable.
The write characteristic can be improved if β(xf) of the transfer transistor is larger than β(pu) of the PMOS transistor. Therefore, there is a request for larger β(xf) also from this viewpoint.
Fine patterning of devices increasingly makes it difficult to eliminate such the tradeoff required for the transfer transistor.